1. Field of the Invention
The application relates to a method utilized in a wireless communication system and a communication device thereof, and more particularly, to a method of performing buffer status reporting in a wireless communication system and a related communication device.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE, MAC provides transmission services via a plurality of logic channels. To manage uplink resources, when a user equipment (UE) performs uplink transmission, the network can divide or map logical channels of the UE into at most four groups based on priorities, types, QoS and so on. The 3rd Generation Partnership Project, 3GPP, introduces a buffer status reporting (BSR) procedure, which is used to provide the network with information about an amount of data in at least an uplink buffer (hereafter called a buffer size) of the UE.
In the BSR procedure, UE uses a MAC PDU (Protocol Data Unit) to carry a BSR MAC control element for reporting buffer size of one or four logical channel groups of the UE. Accordingly, the network can determine the total amount of data available across one or four logical channel groups, so as to allocate uplink resource to the UE for data transmission. According to different requirements, BSR MAC control element can be categorized into two formats: short and long formats. A short BSR MAC control element is 1-byte long, and has 8 bits, where the former 2 bits (called a logical channel group identity (LCG ID) field) indicate one logical channel group which buffer status is being reported, and the remaining 6 bits (called an index field) indicate an index value. A long BSR control element is three-byte long, and has four index fields indicating four index values at a time.
More specifically, please refer to FIG. 1, which is a schematic diagram of a buffer size table according to the prior art. According to the buffer size table, each index value (e.g. index=‘0’-‘63’) corresponds to a buffer size value (e.g. from 0 to 150000 byte) in byte. For example, the index=‘0’ corresponds to BS=0, the index=‘1’ corresponds to 0<BS<=10, and so on. Thus, when the network receives the MAC PDU including the BRS MAC control element, the network knows the buffer status of the logical channel group according to the index value in the index field of the BSR MAC control element and the buffer size table, so as to allocate sufficient resource to the UE.
Moreover, the MAC PDU carries a MAC header consisting of at least a MAC subheader. The MAC subheader corresponding to the BSR MAC control element generally consists of two Reserved bits, an 1-bit Extension field and a 5-bit logic channel identity (LCID) field. In the current specification, the format (e.g. the short BSR MAC control element or long BSR MAC control element) of the BSR MAC control element can be identified by a LCID value in the LCID field.
A long term evolution-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at a cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (CoMP), UL multiple-input multiple-output (MIMO), etc.
For bandwidth extension, carrier aggregation is introduced to the LTE-Advanced system for extension to wider bandwidth, where two or more component carriers are aggregated, for supporting wider transmission bandwidths (for example up to 100 MHz) and for spectrum aggregation. According to carrier aggregation capability, multiple component carriers are aggregated into overall wider bandwidth, where the UE can establish multiple links corresponding to the multiple component carriers for simultaneously receiving and/or transmitting.
In the LTE system, the buffer size table of FIG. 1 is defined for the UE supporting a single component carrier. For the UE supporting only one component carrier, the maximum buffer size value (e.g. 150000 bytes) corresponding to the maximum index value (e.g. index=‘63’) is sufficient for buffer status reporting. However, in the LTE-Advanced system, the UE is allowed to transmit/receive data on multiple component carriers to increase the data rate. Thus, the maximum buffer size value is not enough for buffer status reporting in multiple component carriers, causing the network allocates insufficient resource to the UE for data transmission. For example, there is 200000 bytes in an uplink buffer for transmission. The UE sends a MAC PDU including a BSR MAC control element to the network (i.e. an eNB) for buffer status reporting. However, the BSR MAC control element indicates index=‘63’ to the eNB, and thereby the eNB may think there is only 150000 bytes for uplink transmission. Thus, the eNB may allocate insufficient resource to the UE.
In addition, an extended buffer size table (e.g. 8-bit, or 10-bit buffer size table) may be introduced in the LTE-Advanced system, for extending the buffer size indication range (e.g. up to 3000000 bytes). However, how to performing buffer status reporting is never concerted. In other word, how the MAC PDU for buffer status reporting is not clearly defined. For example, due to the conventional format of the BSR MAC control element (e.g. only 6 bits for index value indication), the index value indicated by the BSR MAC control element is up to index=‘63’. In a word, even though there is an index value in the extended buffer size table larger than index=‘63’, the current BSR MAC control element is unable to indicate the extended index value (e.g. index=‘255’ with 8-bit buffer size table) or extended buffer size value (e.g. 3000000 bytes).